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The hydrogeochemical and stable isotopes of water (δD and δ18O) were combined to investigate the deep groundwater recharge processes in the geologically complex intermountain basin (Kathmandu Valley). Results of the stable isotopic composition of the deep groundwater compared with global and local meteoric water lines and d-excess showed the deep groundwater as a meteoric water origin which is insignificantly affected by evaporation. The analysis suggests the deep groundwater was recharged during high rainfall periods (wet season). Additionally, the control of seasonal variation was absent in the deep groundwater and in the spring water samples. The large range of isotopic composition distribution was due to the altitude affect, whereas variations are from the various geological settings of the infiltration encountered during the recharge processes. The tri-linear diagram showed Na-K-HCO3 and Ca-Mg-HCO3 as the two major water types. The distribution of water types in this intermountain basin was found to be unique compared to other basins. Ionic concentration of the samples was found to be higher in the central part than in the periphery due to the ion exchange processes. This study determines the spatial distribution of various recharge processes that depends upon the environment during rainfall and the geological settings.

Study of flow pattern and water level fluctuation in the western part of the Chitwan Valley was carried out in the area of 274.48 km2. The study area consists of alluvial deposits of the Narayani and the Rapti River of Pleistocene to Recent age. Aquifers in the study area are unconfined and semi confined to confined types. Based on the static water level, Jagatpur, Sukranagar and Gunjanagar are deep tube well zone and Shivanagar, Parbatipur, Patihani, Gitanagar are shallow tube well zone. The western half area discharges groundwater to the Narayani River and southern half discharges to the Rapti River rather than getting recharged throughout the year. Average water level fluctuation is 2.34 m with high fluctuation of 4.88 m at Sukranagar and low 0.49 m at Dibyanagar VDC. This fluctuation is directly related to rainfall.

Groundwater supply is an essential component in any disaster preparedness plan for the Kathmandu valley. Currently, groundwater supply contributes about 40% of the total urban water supply in the Kathmandu valley. In the event of natural disasters like earthquakes, groundwater may be the only water resource available. Considering the existing conditions and the facilities available at the abstraction points, however, reliability of this resource being extracted and put into use in the post disaster period however needs some serious planning and preparedness. Currently though large number of groundwater abstraction points are available in the vicinity of the the proposed Internal Displacement Person (IDP) sites, most of the sites do not have any preparedness plan in case of disaster.

The increasing concentration of nitrogen compounds in the groundwater is of a growing concern in terms of human health and groundwater quality. Although an excess of nitrogen compounds in the groundwater of the Kathmandu Valley has been reported, the seasonal variations of the fate of the nitrogen compounds and their relationships to the subsurface sediments are unknown. In this study, spatially distributed shallow dug well samples were collected during both the dry and wet seasons of 2016, and the nitrogen compound, chloride (Cl&minus, ), and iron (Fe2+) concentrations were analyzed. Two shallow dug wells and one deep tube well were monitored monthly for 2 years. Although NH4-N concentrations were similar in the clay-dominated areas during both seasons (1 and 0.9 mg-N/L), they were lower in the gravel-dominated areas during wet season (1.8 &gt, 0.6 mg-N/L). The NO3-N concentration differed depending upon the soil type which increased during the wet season (clay 4.9 &lt, 13.6 mg-N/L and gravel 2.5 &lt, 6.8 mg-N/L). The Fe2+ concentration, however, was low during the wet season (clay 2.7 &gt, 0.4 mg/L and gravel 2.8 &gt, 0.3 mg/L). Long-term analysis showed higher fluctuation of nitrogen compounds in the gravel-bearing areas than in the clay-bearing areas.

The civilization of Kathmandu Valley has started alongside the holy Bagmati River. However, rapid urban expansion and overpopulation have resulted not only in water shortage but ended up polluting the same water body based on which the settlements had grown, a classic example can be taken as that of Kathmandu and other surrounding cities in Kathmandu Valley. Due to resource availability and centralized government system, many people have migrated to the Kathmandu in recent decades. The annual population growth rate in the valley is 4.63% (CBS 2011) which has created haphazard urbanization resulting in water supply challenges. Geologically, Kathmandu Valley is an intermountain bowl-shaped basin comprised of both shallow and deep aquifers composed of fluvio-lacustrine sediments. Sandy gravel layers of northern side of the valley are considered the recharge zone due to the presence of unconsolidated coarse-grained deposits. Unfortunately, the area has the fastest urban growth and surface sealing, resulting in decrease in natural infiltration. Kathmandu Upatyaka Khanepani Limited (KUKL), the only one organization facilitated by the government, provides approximately 25–33% of the total demand of 350 MLD. Analyses have shown that there will not be water shortage between 2023 and 2025 if the Melamchi Water supply Project (MWSP) is completed within the allocated time. However, even that would not quench the thirst of the people as the demand is expected to rise in coming years. Proper planning, good governance, identification of water sources, and water treatment of wastewater are the long-term solutions for sufficient water supply. Alternate mitigation options, such as proper use of groundwater and surface water and spring water management with appropriate distribution system, would be useful. Unless the problem is tackled tactfully from different aspects and not only from demand supply and soon, the increasing social conflicts could be challenging for the planners in the future.

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Elevated arsenic in groundwater affects some 100 million people in South Asia, yet mitigation efforts are hindered by persistent uncertainty about the proximal source of arsenic and mechanisms for its mobilization. At the core of this uncertainty are the relative roles of surficial organic clays vs. deeper aquifer matrix iron oxyhydroxides. Temporal variations in groundwater chemistry can serve to distinguish the contributions of these two sources, and such variation is especially pronounced in headwater areas of the Ganges floodplain immediately adjacent to the Himalayan foothills (e.g. the Terai of Nepal). Tubewells down to 50 m in the Terai commonly exhibit cyclical, temporally-correlated variation in dissolved arsenic, iron and other species. In Nawalparasi, the most arsenic-affected district, these wells tap thin (2 m) gray sand aquifers embedded in a thick (>50 m) sequence of organic clays. Monsoon recharge refreshes these aquifers, temporarily minimizing arsenic concentrations. Post-monsoon, average groundwater compositions exhibit increasing trends in water–rock interaction (higher TDS, with cation exchange to form increasingly Na–HCO 3 waters), arsenic and iron. This cycle can be repeated during dry-season precipitation events as well, revealing direct correlation between trends in degree of clay interaction (sodium fraction of major cations) and arsenic concentrations. During the year, reversals in vertical head gradient yield reversals in arsenic temporal trend, and downward gradients in the dry season correlate with increases in arsenic. Collectively these observations strongly support a model of reductive mobilization of arsenic from adjacent clays into aquifers, tempered by repeated flushing during periods of appreciable rainfall.

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Mechanical and physical characteristics of rocks generally depend on their composition and texture, because they are reflective of their environments during sedimentation, diagenesis and weathering in each location. To understand this, relationships between mechanical and physical indices and composition and texture have been analyzed using several rock samples obtained from Mio-Pleistocene sandstone sequence of the Siwalik Group, widely distributed in foothills of the Nepal Himalayas. Sandstones are quartzo-feldspathic to quartzo-lithic and quartzo-feldspathilithic. Cement, matrix and void vary from 6 to 41%, 0 to 18% and 1 to 12%, respectively. Ferroan calcitic and siliceous cements are the dominant cements, and their ratio to total cement varies from 0.2 to 1.0. Packing density of sandstones ranges from 59 to 95%. Clasts are loosely to closely packed and are weakly to strongly bonded by cement. Results of correlation analyses show that sandstones strongly depend on several indices, that is percent void, strong cement over total cement, strong over weak contacts, concavo–convex contacts and packing density, most of which are measured on thin sections. Contrarily, they depend little on grain size, shape, mineralogical maturity index and the other petrographic indices. Increased interlocking and strong cementing with reduced void tends to possess high densities, low porosity and offer good resistance against deformation. Therefore, proportion of strong cement over total cement, void content, strong over weak contacts, grain-to-void contact, concavo–convex contact and packing density are meaningful indices considering the sandstones of the Siwalik Group. Based on the multiple regression, correlation analyses and then stepwise multiple regression analysis, some regression equations with significant explanatory indices are indicated for estimating mechanical and physical properties. In other words, such relationships may be useful in estimating mechanical and physical properties using the significant petrographic indices.

Occurrence of arsenic in shallow aquifers was studied from the Nawalparasi district in west Nepal. A higher concentration of arsenic was found in the wells from the north as compared to those from the south. The arsenic level in the north reaches a maximum of 694ppb as compared to a value of 27 ppb in the south near the Nepal-India border. The arsenic concentration analyses carried out in selected sites from March to September 2003 indicate a large variation (exceeding 200%) in the north as compared to the central and southern regions. A general increase in grain size from north to south was observed in the well logs. Generally, fine sediments like clay and silt constitute more than 80% of the drilled depth in the north (i.e., at Panchnagar), while the fines are about 32% in the south (i.e., at Bhujawa). This type of grain size distribution is in contrast to the generally observed fining-southwards pattern in the Terai.

The Asian Arsenic Crisis has expanded into the headwaters of the Ganges River, now including the plains (Terai) of Nepal. This study seeks a non-invasive predictive tool to estimate groundwater arsenic concentration prior to drilling, enabling "arsenic avoidance" in contaminated areas. Detailed chemical studies indicate that in Himalayan-sourced aquifers arsenic is released by microbially-mediated redox reactions. Likely hydrogeological settings for oxidizing chemical conditions (immobile arsenic) should be more porous (higher in filtration rate for oxygenated waters) and contain fewer fine organic sediments (oxygen-consuming material). Both conditions should yield higher electrical resistivity, and such aquifer heterogeneity effects should be most prominent in head water regions such as Nepal. To test this approach, a series of vertical electrical resistivity soundings were made near Parasi, Nepal, constituting a profile extending 2 km across a known high-arsenic area. Correlation of the horizontal and vertical distribution of measured resistivity and ENPHO groundwater arsenic measurements demonstrated a distinct inverse relationship between these variables. Out of 240 arsenic sample points, 75% of those extracted from high resistivity zones (>100 ohm-m, inferred lower clay content) exhibited arsenic 150 ppb. Given these preliminary results, the resistivity technique appears to hold great promise as a predictive tool for finding low-arsenic groundwater zones within contaminated areas, thereby allowing "well-switching" from highly toxic to new safe or more readily treatable wells. The method should be applicable in most circum- Himalayan high-arsenic areas.

Middle Miocene to early Pleistocene sedimentary sequence deposited in the foreland basin of the Himalaya is represented by the Siwalik Group. In the present study area the Siwalik Group extends in a NW-SE direction and well-exposed. Forty­four sandstone samples were studied for texture, fabric and composition in order to assess their petrographic properties and variation trends of these properties in stratigraphic levels. Sandstones were classified into sublitharenite, subarkose, lithic arenite, arkosic arenite and feldspathic graywacke and further thirteen sub-clans. Mean grain size (M) and Trask sorting coefficient (So) increase up-section. Recalculated quartz, matrix, modified maturity index (MMI), total cement (Ct), cement versus matrix index (CMI) and ratio of strong cement over total cement ((Cfc/Cs)/Ct) also increase, whilst packing proximity (PP), packing density (PD) and consolidation factor (Pcc) decrease up-section showing distinct trends, and therefore, these properties are promising in recognizing the older sandstones from the younger ones.

Core drillings in ancient lake sediments of the Kathmandu Valley, central Nepal Himalaya were carried out in order to clarify the past Indian monsoon climate and its linkage to the uplift of the Himalaya. This is the first large-scale drilling project in the valley with full core recovery, and solely dedicated to academic research purpose. The drilling penetrated the whole sequence of 208 m thick Kalimati Clay, newly defined as the Kalimati Formation. Based on the drill-core study, the sediments are divided into three formations: 1) Bagmati Formation, 2) Kalimati Formation, and 3) Patan Formation in ascending order. These formations have very distinct sedimentary characteristics: Bagmati essentially gravelly, Kalimati clayey and Patan sandy. The lower part of the Kalimati Formation, showing marginal lake facies, has been designated as the Basal Lignite Member. Judging from the lithology and sedimentary facies and previous studies on the Lukundol Formation, the Bagmati Formation is interpreted to have been deposited by the Proto-Bagmati River prior to the appearance of the lake before 2 Ma. The clay-predominant Kalimati Formation lying over the Lignite Member was deposited in an open lacustrine environment, which is mainly composed of carbonaceous clay yielding abundant fossil leaves and diatomaceous laminite. The fluvial sand of the Patan Formation rests on the Kalimati clay bed with a marked erosional base. The 14C age of the uppermost part of-the Kalimati Formation suggests that the lake water of the Kathmandu Basin was drained out later than 10 kyr B. P. (Fujii and Sakai 2002). Based on comparison of the present altitude of the ancient lake floor and top set beds of the lacustrine delta sediments (dated as 29 kyr B.P.), the water-depth of the Palaeo-Kathmandu Lake in the central part of the valley is estimated to be about 70 m.

The Chitwan Valley is one of the largest Dun Valleys in the Himalayan foothills of Nepal. Dun gravels perhaps deposited in the late Pleistocene to very early Holocene about 22,000-7000 yr. B.P. Chitwan Dun Valley is underlain by Dun fan gravels or Dun gravels which form unconfined to semiconfined or leaky confined aquifers. The study area, situated in the NW part of the valley occupies an area of 70.8 km2.The hydrogeological situation in the study area is inferred from drilling data of Ground Water Resources Development Board (GWRDB), Agriculture Development Project Janakpur (ADPJ) and several private drilling companies .The Chitwan Dun Valley constitutes a closed groundwater system in the Siwalik Zone of Nepal Himalaya. The study area reveals the existence of two definite groundwater sub-basin each having its own hydraulic system and is a part of single large regional groundwater basin. Annual potential evapotranspiration (PET) calculated at Rampur (station no 0902) for five-year period (1990-1995) is 1.68 mm/day. Annual precipitation data recorded at Rampur is 2214 mm. Area of recharge is 70.8 km2 and estimation of total groundwater storage(reserve) is 87.31 MCM per year, and dynamic reserve or annual potential recharge is estimated as 48.60 MCM per year. doi: 10.3126/bdg.v12i0.2249 Bulletin of the Department of Geology, Vol. 12, 2009, pp. 43-54